This invention relates to a power supply circuit for driving a magnetron and more particularly to a power supply circuit which supplies electric power at wide conduction angles for driving a magnetron in a microwave heating apparatus such as a microwave oven at a higher frequency than a commercial frequency.
Conventional power supply circuits for a magnetron in a microwave heating apparatus comprise a transformer for 50 or 60 Hz, condensers and diodes. Recently methods to drive a magnetron at a high frequency in a microwave heating apparatus, for example, a microwave oven have been studied in order to make the apparatus lightweight and compact, and to facilitate the control of its power output by using an inverter power supply. The Japanese Patent Application Laid-open Publication No. 60-250588 discloses a microwave oven which includes means for supplying a constant power to a heater of a magnetron, which is connected to a secondary winding of a transformer in an inverter circuit, and means for controlling the output power by switching a switching element on and off in the inverter circuit and for maintaining the output below a predetermined value for a predetermined time just after power-on.
Employment of a high frequency obtained with an inverter can make electrical parts considerably small and lightweight, resulting in a compact and lightweight microwave heating apparatus, but it leaves much to be desired for its commercialization. One of the problems is that a magnetron conducts little before a voltage Vm across its anode and filament reaches 3.5 KV, for example and conducts abruptly after that as shown in FIG. 1, this resulting in difficulties in the control of power outlet. For these reasons a power supply employing an inverter circuit was not compatible with a magnetron.